Modern jet aircraft include a multitude of onboard electrical and pneumatic systems. The power needed to run these systems is generated by operation of the aircraft's jet engines while the aircraft is in flight. When the aircraft is parked at a terminal to load or unload passengers, however, the jet engines are turned off. During these periods, many of the electrical and pneumatic systems onboard the aircraft may, nevertheless, need to be operated. To accommodate such needs for continual power, an auxiliary power unit (hereinafter “APU”) is commonly mounted to the aircraft. An APU is essentially an additional jet engine that does not provide any significant thrust for the aircraft during flight, but which can be operated while the aircraft is on the ground (and also while in flight) to generate electricity for the aircraft's electrical systems and provide air to the aircraft Environmental Control Units or ECU's.
The APU is commonly mounted in the tail cone of the aircraft and has an exhaust system that vents out of the rear of the tail cone. When the APU is operated, it emits a very loud noise which, if not muffled, could be an irritant to the members of the ground crew. To reduce the volume of the noise produced by the APU, the APU is commonly fitted with a muffler. The muffler is placed around the APU's exhaust pipe so that all of the APU's exhaust (both noise and gas) are channeled through the muffler. The muffler is designed and constructed to substantially reduce the volume of the noise emitted by the APU.
The muffler is made of metal and has a tendency to get very hot during operation of the APU because of the temperature of the exhaust gasses generated by the APU. It has been observed that the temperatures on the outer skin of the muffler commonly reach above 1,000 degrees Fahrenheit. If the muffler is not insulated, this heat will radiate outwardly from the muffler to the tail cone. Modern aircraft tail cones are commonly made from composite materials and plastics to help keep the overall weight of the aircraft low. Such materials cannot tolerate the high temperatures radiating from the muffler and if exposed to such temperatures for any length of time, may experience some form of failure. Accordingly, aircraft manufactures commonly mandate that the heat radiating from the muffler not exceed a predetermined limit.
To accommodate this, mufflers are commonly fitted with an insulating assembly that substantially encloses the muffler and obstructs the heat from radiating outwardly from the muffler. The insulating assemblies are generally blankets of flexible insulating material that are shrouded in an outer foil shell. The insulating assemblies are commonly constructed from two such blankets, one of which is wrapped around a top portion of the muffler and the other of which is wrapped around the bottom portion of the muffler. Once the two blankets are in place around the muffler, they are attached to one another via any of a variety of suitable fastening means.
The blankets that are used to construct the insulating assembly are typically very dense because of the temperatures that they need to resist. Consequently, the typical insulating assembly is relatively very heavy. Because of the high cost of fuel and because of the direct correlation between overall aircraft weight and fuel consumption, it is desirable to redesign existing insulating assemblies to reduce their weight and thereby reduce the overall weight and rate of fuel consumption of the aircraft. Such weight reductions, however, should be accomplished without compromising the insulating assembly's ability to ensure that the heat that radiates from the muffler remains below the manufacturer's predetermined threshold.